ABOUT HUMIDITY
What Humidity Is
Humidity is the moisture carried or held in the air. It is actually dissolved in the air, and that's why you can't see it even though you can feel it. If there is more moisture than the air can carry, the excess moisture becomes visible as condensation. The condensate may remain suspended in the air (as in fog or clouds or steam from your kettle) or, if it is heavy enough, it will fall to the ground (as rain or dew).
How much moisture the air can carry depends, among other things, on the temperature of the air. The warmer the air, the more moisture it can hold. The usual way to indicate humidity is to refer to the relative humidity. Relative humidity is a measure of how much moisture the air is actually carrying compared to how much it could carry. Air at 50% relative humidity is holding half of the moisture it could hold at 100% humidity.
Relative humidity can be confusing because it feels different at different temperatures. Warm air at 50% relative humidity is carrying more moisture than cold air at 50% humidity. In both cases, the air is holding half of its moisture-carrying capacity, but since the warm air can hold more moisture, it feels more moist than the cold air, which is holding less moisture.
Now for the tricky part. If you take warm air holding a certain amount of moisture, and then cool that air down, its relative humidity will go way up. Even though the air is still holding the same amount of moisture, its carrying capacity is lower because it is cooler, so the air is relatively more humid. This is what happens in the summer when the humid, warm air drifts down to the basement and cools off. The relative humidity goes way up, and the basement feels dank.
In the winter, there is the opposite problem. The cold air enters your house at a certain relative humidity. Then your heating system warms the air up. This causes the relative humidity to drop. Even though the air is still holding the same amount of moisture, its carrying capacity is higher because it is warmer, so the air is relatively less humid. So everything in the house, including your skin, dries out.
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What It Does To Your Piano
Because the air permeates all the wood in a piano, the wood is moistened by the air's humidity. If the air's humidity increases, the wood is further moistened, and the wood will swell. If the humidity decreases, the wood will dry out and shrink. If the piano is in a basement, its wood will swell like crazy in the summer. If the piano is in a heated room, its wood will shrink a lot in the winter.
A certain amount of shrinkage and swelling is acceptable, but too much will cause damage. Too much humidity causes the most damage. Too little humidity (too much dryness) will cause damage. And cycling back and forth during the year from too much to too little humidity will age the piano prematurely and wear it out.
Here's what happens in detail:
Too Much Humidity in Summer: The soundboard swells, causing the piano's pitch to rise; wooden parts joined in place with screws or glue swell up and crush themselves against the joint; wooden parts warp and bind as they swell; all the glue joints weaken as the glue absorbs the moisture; hammer felts swell and soften, affecting the tone; mildew grows, and strings rust.
Too Little Humidity in Winter: The soundboard shrinks, causing the piano's pitch to drop; wooden parts, crushed in the summer, shrink and become loose; wooden parts warp, bind, and crack as they shrink; weakened glue joints let go; hammer felts shrink and harden, affecting the tone.
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What You Can Do to Help Your Piano
The best thing you can do is reduce the extremes of humidity, especially high humidity in the summer. Use air-conditioning to dehumidify the air - you don't have to bring the room's temperature down a lot, use it just enough to dry the air a bit. If you must keep a piano in the basement (or the first floor of a masonry building), use a dehumidifier for the room.
In the winter, try not to heat the piano room too much - the cooler the better. Move the piano away from heat sources - even a little bit of space will make a difference. Use a room humidifier if necessary.
You can purchase inexpensive humidity gauges, but try not to "aim" for a single humidity level all year. Expect the humidity to go up and down - the piano can handle normal fluctuations. Learn to read the extremes, and adjust for them.
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